An Isotope-labeling Study of CO₂ Formation Pathways in Molecular Clouds: Ultraviolet Photolysis of CO–H₂O Ice

Abstract With the use of reflection–absorption infrared spectroscopy, we investigated the binary ice of CO and H 2 O at 10 K under ultraviolet irradiation as an analog of the interstellar ice mantle. We employed H 2 18 O instead of H 2 16 O to distinguish the formation pathways of CO 2 , and prepared mixed-ice samples under CO- and H 2 O-rich conditions as well as bilayered ice, where CO was condensed on H 2 O ice. After the ultraviolet irradiation onto any sample, we detected infrared absorption bands of C 16 O 2 and C 18 O 16 O, which were generated through the CO–CO and CO–H 2 O reactions, respectively. The additional detection of C 18 O 2 clearly indicated that the photodissociation and regeneration of CO 2 also took place in ice. We analyzed the irradiation-time dependence of the C 16 O 2 and C 18 O 16 O column densities in CO-rich ice to determine the effective cross sections of the CO 2 formation through the CO–CO ( σ C 16 O 2 ) and CO–H 2 O ( σ C 18 O 16 O ) reactions simultaneously: σ C 16 O 2 = 3 × 10 −21 cm 2 and σ C 18 O 16 O = 5 × 10 −18 cm 2 . Despite the rather small value of σ C 16 O 2 compared to σ C 18 O 16 O , the former reaction was found to be nonnegligible in terms of the CO 2 yield, even at the H 2 O-rich condition.